1. Technical Field
The present invention generally relates to a method for writing a grating in a large diameter optical waveguide; and more particularly to a method for writing a grating in a large diameter optical waveguide in which photosensitivity is restored or increased in a hydrogen or Deuterium loaded large diameter optical waveguide.
2. Description of Related Art
It is known in the art that the presence of hydrogen, Deuterium or other suitable photosensitizing gases in a germanium doped waveguide enhances the photosensitivity of that waveguide. This is well documented in numerous references such as, “Photosensitive changes in Ge-doped fibers observed by Raman spectroscopy,” D. McStay, SPIE vol. 1315 Fiber Optics '90, pp. 223–233; “Permanent photoinduced birefringence in a Ge-doped fiber,” Francois Ouelette, Daniel Gagon, Michel Poirer, Applied Physics Letters, vol. 58, pp. 1813–1815, 1991; U.S. Pat. No. 5,235,659 “Method of making an article comprising an optical waveguide,” Robert M. Atkins, Paul J. Lemaire, Victor Mizrahi, Kenneth L. Walker; Aug. 10, 1993; U.S. Pat. No. 5,287,427 “Method of making an article comprising an optical component, and article comprising the component,” Robert M. Atkins, Paul J. Lemaire, Victor Mizrahi, Kenneth L. Walker; Feb. 15, 1994.
These prior art descriptions focus primarily on methods for increasing the photosensitivity of fiber. Fiber has several unique characteristics. Optical fiber is typically coated with an organic polymer that cannot withstand high temperatures. Single mode optical fiber is also typically 80 or 125 microns in diameter. These characteristics drive the method by which the above references incorporate hydrogen into the optical fiber. In particular, very high temperatures are not employed, as this would damage the optical fiber coating. However, low temperatures limit the diffusion rate at which hydrogen is incorporated into the glass. For 125 micron optical fiber, this is not a terrible problem as at temperatures between 50E C and 80E C (well below the damage temperatures for most fiber coatings) significant hydrogen can be diffused into the fiber in a reasonable time frame (less than 1 day). However, for significantly larger glass structures the time quickly becomes excessive.
Photosensitivity requirements can only be analyzed qualitatively at present. It was recognized that the photosensitivity requirements, for strong or multiple collocated gratings, were not met in initial H2 loading iterations. In the case of a 2.5 mm diameter Waveguide, the combination of a low photosensitivity waveguide and a concentration of −1.44×10^20 ions/cm^3 of H2 were not sufficient to allow two gratings to be collocated without undesirable out-of-band spectral characteristics. To add a third collocated grating, it was necessary to increase the pressure by a factor of 6 (as well as reducing the temperature by 25E C to increase solubility). The resulting H2 concentration increased to −5.47×10^20 ions/cm^3 This higher concentration has proven to be sufficient to allow collocating three gratings. However, the time required to reach 95% saturation, 21 days, is excessive and must be reduced to achieve a reasonable cycle time for volume production of collocated gratings in large waveguides. The next logical step will be to determine the effects of raising the temperature by 15–20% (absolute), to 250–275E C, which would reduce the loading time to 3–4 days with a resulting concentration of 3.46×10^20 ions/cm^3. Provided that there is still sufficient photosensitivity after the solubility losses, an additional step would be to determine if the pressure could also be reduced without causing the sideband issue to reoccur when collocating gratings.
For cylindrical geometries of 2000 microns, the time to reach a reasonable diffusion equilibrium is many days, perhaps even weeks at low temperatures.